Someone (male or female) that is so bad at sex you may as well be fucking a corpse, although a corpse may sometimes be more lively. These types are usually fairly easy to spot by the lack of personality. They are usually followers.

Female dead lays: from beginning to end, she just lays there, no expression or movement, unless it's that of boredom. You may want to check in between to see if she's awake. ;)

Male dead lays: Usually those that are too lazy to put any effort into it. Kinda like a quickie, but with only one person putting any effort into it. Most "nice guys" fit into this category.